Heater arrangement for crystal growth furnace

ABSTRACT

A furnace for growing a high volume of crystals includes a plurality of individual growth stations and first and second heater matrixes. Each individual growth station has a crucible and an insulating container generally surrounding the crucible and thermally isolating the crucible from the other individual growth stations. The first and second heater matrices each include at least two legs electrically connected in parallel and each of the legs have at least two resistance heaters electrically connected in series. Each of the individual growth stations have at least one of the resistance heaters within the first heater matrix and at least one of the resistance heaters within the second heater matrix associated therewith. The resistance heaters of the first heater matrix are located above the crucibles and are preferably adapted to provide a homogeneous temperature across tops of the crucibles. The resistance heaters of the second heater matrix are preferably located below the crucible and are preferably adapted to provide a temperature gradient across bottoms of the crucibles.

This application is a continuation of application Ser. No. 09/349,597,filed Jul. 9, 1999, now U.S. Pat. No. 6,537,372 which application claimspriority of provisional application Serial No. 60/141,389, filed Jun.29, 1999.

BACKGROUND OF THE INVENTION

The present invention generally relates to a heater arrangement for afurnace and, more specifically, to a heater arrangement for a crystalgrowth furnace suitable for producing a high volume of crystals.

Furnaces for the production of crystals, such as single crystals ofcalcium fluoride, typically have a crucible which is loaded with a seedand/or starting material. A heater (or heaters) is arranged about thecrucible to produce a temperature gradient to grow the crystals in thecrucible. Growth is obtained by varying power to the heater according toan established power-temperature relationship to obtain the desiredthermal environment.

The thermal gradient obtained is critical to growing a single crystalrather than a polycrystalline structures. Additionally, the quality of asingle crystal is believed to be primarily affected by the appliedthermal gradient. Present furnaces for the production of macrocrystals,therefore, have elaborate and complex heaters and/or controllers forcontrolling the heaters to obtain the desired thermal environment. Thesecomplex devices are expensive to produce and complicated to operate andmaintain.

In high volume production of crystals it is important to obtain thedesired thermal environment and is also important to get consistenttemperature environments. Accordingly, there is a need in the art for acrystal growth furnace which is simple to produce and operate, producesdesired thermal environments for growing crystals, and producesconsistent thermal environments for growing a high volume of crystals.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a furnace for growing crystals whichovercomes at least some of the above-noted problems of the prior art.According to the present invention, a heater arrangement for a crystalgrowth furnace includes a plurality of individual growth stations eachhaving a crucible. The crystal growth furnace also includes a firstheater matrix having at least two resistance heaters electricallyconnected in series or parallel. Each of the individual growth stationshas at least one of the resistance heaters of the first heater matrixassociated therewith and located near the crucible. By connecting theresistance heaters of separate growth stations in this manner, thetemperatures produced by the resistance heaters in the separate growthstations are fixed at the same temperature for a given power level whenthe resistance heaters are connected to a single power source.

According to another aspect of the present invention, a heaterarrangement for growing crystals includes a plurality of individualgrowth stations each having a crucible. The heater arrangement alsoincludes a first heater matrix and a second heater matrix separate fromthe first heater matrix. Each heater matrix preferably includes at leasttwo legs electrically connected in parallel with each of the legs havingat least two resistance heaters electrically connected in series. Eachof the individual growth stations has at least one of the resistanceheaters of the first heater matrix and at least one of the resistanceheaters of the second heater matrix associated therewith. By having twoseparate heater matrices, the temperatures produced by the resistanceheaters in a large quantity of separate growth stations can be fixed atthe same temperature for a given power level yet the temperaturegradient formed in each of the growth stations can be varied when eachheater matrix is connected to a separate power source. Preferably, theresistance heaters within the first heater matrix are located above thecrucibles and provide a homogeneous temperature across the top of thecrucibles and the resistance heaters within the second heater matrix arelocated below the crucibles and provide a temperature gradient acrossthe bottom of the crucibles.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

These and further features of the present invention will be apparentwith reference to the following description and drawings, wherein:

FIG. 1 is a perspective view of a crystal growth furnace having aplurality of individual growth stations for producing a high volume ofmacrocrystals according to the present invention and with components,such as a vacuum chamber, removed for clarity;

FIG. 2 is a perspective view of the crystal growth furnace of FIG. 1with further components, such as crucibles, removed for clarity to showa heater matrix;

FIG. 3 is a plan view of an upper resistance heater of the arrangementof FIG. 2;

FIG. 4 is a plan view of an lower resistance heater of the arrangementof FIG. 2;

FIG. 5 is an enlarged elevational view, in cross-section, of onecrucible of the crystal growth furnace of FIG. 1;

FIG. 6 is a perspective view of a crystal growth furnace having aplurality of individual growth stations for producing a high volume ofmacrocrystals according to a second embodiment of the present inventionwith components, such as a vacuum chamber, removed for clarity;

FIG. 7 is a perspective view of a crystal growth furnace having aplurality of individual growth stations for producing a high volume ofmacrocrystals according to a third embodiment of the present inventionwith components, such as a vacuum chamber and crucibles, removed forclarity to show a heater matrix;

FIG. 8 is a perspective view of a crystal growth furnace having aplurality of individual growth stations for producing a high volume ofmacrocrystals according to a fourth embodiment of the present inventionwith components, such as a vacuum chamber and crucibles, removed forclarity to show a heater arrangement; and

FIG. 9 is a plan view similar to FIGS. 3 and 4 but showing analternative heater leg.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a heat treatment or vacuum furnace 10 according tothe present invention suitable for growing a high volume of crystals.The crystal growth furnace 10 can be used to grow low and hightemperature crystals using, for example, melt, Vapor Phase Epitaxy(including VPE, MVPE and OMVPE), Chemical Vapor Deposition (CVD) andthin film processes. The crystal furnace 10 can be used to grow crystalsof a variety of different materials such as calcium fluoride, sodiumiodide, and cesium iodide etc. The crystal growth furnace 10 isparticularly useful in growing single macrocrystals having diameters of,for example, up to about four feet and larger but can also be used togrow other types of crystals such as microcrystals or polycrystallinestructures.

As best shown in FIGS. 1 and 5, the crystal growth furnace 10 includes awater-cooled pressure vessel or vacuum chamber 12 and a plurality ofindividual growth stations 14 located within the vacuum chamber 12. Thevacuum chamber 12 preferably operates at a pressure of 10 millitorr orless. The dimensions of the vacuum chamber 12 depend on the size andnumber of individual growth stations 14 located therein. In theillustrated embodiment there are four individual growth stations but agreater or lesser number can be utilized as discussed in more detailhereinbelow.

Each individual growth station 14 includes a crucible 16 in which acrystal will grow, an insulating container or cylinder 18 surroundingthe crucible 16, and two heaters 20, 22 located within the insulatingcylinder 18 and adjacent the crucible 16. Preferably, a top heater 20 islocated above the crucible 16 and a bottom heater 22 is located belowthe crucible 16. It is noted, however, that other heater locations suchas around the sides of the crucible 18 are within the scope of thepresent invention.

The illustrated crucible 16 is cylindrical or cup-shaped having a sidewall 24 and a bottom wall 26. The crucible 16 is formed of a suitablematerial such as, for example, graphite, carbon, or other carboncomposites. The dimensions of the crucible 16 depend on the desired sizeof the crystal to be grown therein. A crucible believed suitable forgrowing single crystals of calcium fluoride has an outer diameter ofabout 5.5 inches, an outer height of about 12 inches, an inner diameterof about 4 inches, and an inner height of about 10.75 inches. Suitableranges for the crucible dimensions are believed to be an outer diameterof from about 2 inches to about 40 inches, an outer height of from about6 inches to about 40 inches, an inner diameter of from about 1 inch toabout 38 inches, and an inner height of from about 3 inches to about 36inches.

The top opening of the crucible 16 is closed by a lid or cover 27. Thelid 27 is formed of a suitable material such as graphite or carboncomposite.

The perimeter of the crucible 16 is provided with suitable insulation28. The insulation 28 comprises a suitable material such as rigidizedcarbon felt or rigidized carbon foam. The inner surface of theinsulation 28 preferably has a thin layer of graphite foil or GRAFOILfor increased reflection of the radiation. This thin layer of graphitefoil preferably has a thickness in the range of about {fraction (1/32)}inch to about ⅛ inch. The insulation 28 preferably extends the entireheight of the crucible 16 from the top to the bottom of the side wall24. The insulation 28 preferably extends about 1 to 6 inches from theouter diameter of the crucible side wall 24 but not beyond the outerdiameter of the top and bottom heaters 20, 22. Insulation having athickness of about 1.5 inches is believed suitable for the abovedescribed crucible for growing single crystals of calcium fluoride butit may be thicker or thinner depending on the insulation heat transfercharacteristics. It is noted that the insulation 28 is located at otherlocations about the crucible 16 when the heaters 20, 22 heaters haveother locations.

The crucible 16 is supported by a pedestal 30 downwardly extending fromthe bottom wall 26, through the bottom heater 22, and rests in a cup oradapter. The cup rests on a shaft 32 which extends through the baseplate or bottom wall 34 of the vacuum chamber 12. The pedestal 30 isformed of a suitable material such as, for example, graphite or carboncomposite while the cup is formed of a suitable material such as steelor inconel, and the shaft 32 is formed of a suitable material such, forexample, as stainless steel or inconel. The pedestal 30 extends into orotherwise supports the bottom wall 26 of the crucible 16 and ispreferably provided with an upward facing recess or pocket 35 forholding a seed or starting material. A pocket having a diameter of about{fraction (7/16)} inches, a length of about 1.5 inches and extendinginto the crucible about 1 inch is believed suitable for the abovedescribed crucible to receive starting material for growing singlecrystals of calcium fluoride. The seed pocket diameters described aboveare for crystals having diameters up to about 13 inches, crystals havinglarger diameters should have seeds greater than about 1 inch indiameter.

The insulating cylinder 18 is adapted to generally surround the crucible16 and the top and bottom heaters 20, 22 so that the top and bottomheaters 20, 22 are insulated from the top and bottom heaters 20, 22 ofthe other individual growth stations 14 so that each of the individualgrowth stations 14 will be thermally independent of the others. Theillustrated insulating cylinder 18 has a cylindrically-shaped side wall36, a top wall 38 generally closing the open upper end of the side wall36, and a bottom wall 40 generally closing the lower open end of theside wall 36. The side, top, and bottom walls 36, 38, 40 comprisesuitable insulation material such as rigidized carbon felt or rigidizedcarbon foam. The insulation material is preferably suitable formaintaining a temperature difference of about 1000° C. to about 1300° C.The side wall 36 is sized so that the top and bottom heaters 20, 22 canreside within the inner diameter of the side wall 36 and is providedwith suitable openings 42, 44 for the passage of the top and bottomheaters 20, 22 therethrough. The surface of the inner diameter of theinsulating cylinder 18 preferably has a thin layer of graphite foil orGRAFOIL for increased reflection of the radiation and reduced radiationlosses. This layer of graphite foil is also preferably located on theinner surfaces of the top and bottom walls 38, 40. This layer preferablyhas a thickness in the range of about {fraction (1/32)} inch to about ⅛inch. An insulating cylinder having wall thicknesses of about 2 inches,an inner diameter of about 9 inches, and an interior height of about 16inches is believed suitable for the above described crucible for growingsingle crystals of calcium fluoride. Suitable ranges for the insulatingcylinder dimensions are believed to be wall thicknesses of from about 2inches to about 4 inches, an inner diameter of from about 4 inches toabout 72 inches, and an interior height of from about 6 inches to about48 inches.

The bottom wall 40 of the insulating cylinder 18 is provided with asuitable opening 46 for passage of the pedestal 30 therethrough.Preferably, a base 48 is provided which supports the side, top andbottom walls 36, 38, 40 and positions the walls 36, 38, 40 about thecrucible 16 and the top and bottom heaters 20, 22.

As best shown in FIG. 2, the top heaters 20 of the individual growthstations 14 are suitably connected to form a top or first heater matrix50 and the bottom heaters 22 of the individual growth stations 14 aresuitably connected to form a bottom or second heater matrix 52. Eachheater matrix 50, 52 includes at least one leg 54, 56 of two or more ofthe heaters 20, 22 connected in series or at least two heater legs 54,56 which are connected in parallel. Preferably, each heater matrix 50,52 includes at least two heater legs 54, 56 with each heater leg 54, 56having at least two heaters 20, 22. The illustrated heater matrices 50,52 each have two heater legs 54, 56 of two heaters 20, 22 to accommodatethe four individual growth stations 14. As noted hereinabove, the numberof heater legs 54, 56 and the number of heaters 20, 22 in each heaterleg 54, 56 can be greater or lesser depending on the number ofindividual growth stations 14 desired. Moreover, each heater matrix 50,52 may be two or more heaters 20, 22 only connected in series, onlyconnected in parallel, or a combination of both.

Power is distributed to the heater matrices 50, 52 by way of bus bars58, 60. Preferably, the bus bars 58, 60 are all located at the sameheight near the bottom of the individual growth stations 14. The busbars 58, 60 are formed of a suitable material such as, for example,graphite or carbon composite. Each heater matrix 50, 52 has a separatepair of the bus bars 58, 60 wherein all of the heater legs 54, 56 of theheater matrix 50, 52 are suitably connected in parallel between the pairof bus bars 58, 60.

As best shown in FIG. 5, the heater legs 54, 56 are connected to the busbars 58, 60 and the bus bars 58, 60 are connected to copper feed-throughelectrodes 62, 64 which extend through the bottom wall 34 of the vacuumchamber 12. The bus bars 58, 60 are preferably connected to the copperfeed-through electrodes 62, 64 via graphite electrodes 63, 65. Graphitebus bars having a height of about 1 inch, a width of about 4 inches, anda length of about 16 inches are believed to be suitable for a fourstation furnace having a heater matrix with two legs of two heaters eachand the above described crucibles for growing single crystals of calciumfluoride. Suitable ranges for the bus bars are believed to be a heightof from about 1 inch to about 5 inches, a width of from about 1 inch toabout 5 inches, and a length of from about 6 inches to about 500 inches.

The heater legs 54, 56 are positioned above and below the crucibles 16and pass through the insulating cylinders 18 via the openings 42, 44.The top heater leg 54 is positioned between and spaced apart from thelid 27 of the crucible 16 and the top wall 38 of the insulating cylinder18. The bottom heater leg 56 is positioned between and spaced apart fromthe bottom wall 26 of the crucible 16 and the bottom wall 40 of theinsulating cylinder 18. Spacings of about 0.75 inches from the lid 27 ofthe crucible 16, about 0.5 inches from the top wall 38 of the insulatingcylinder 18, about 1.25 inches from the bottom wall 26 of the crucible16, and about 0.75 inches from the bottom wall 40 of the insulatingcylinder 18 are suitable with the above described crucibles for growingsingle crystals of calcium fluoride. Suitable ranges for the spacingsare believed to be from about 0.25 inches to about 1 inch for thedistance from the lid 27 of the crucible 16, from about 0.5 inches toabout 3 inches for the distance from the top wall 38 of the insulatingcylinder 18, from about 0.25 inches to about 1 inch for the distancefrom the bottom wall 26 of the crucible 16, and from about 0.5 inches toabout 3 inches from the bottom wall 40 of the insulating cylinder 18.

In the illustrated embodiment, vertically extending supports or posts66, 67 are provided between the heater legs 54, 56 and the bus bars 58,60 at the ends of the heater legs 54, 56. The posts 66, 67 both supportthe heater legs 54, 56 and electrically connect the heater legs 54 tothe bus bars 58, 60. It is noted that the heater legs 54, 56 arepreferably self supporting between the posts 66, 67 at the ends of theheater legs 54, 56 but, if necessary, intermediate posts can beutilized. The posts 66, 67 are formed of a suitable material such as,for example, graphite or carbon composite.

The posts 66, 67 are sized to position the heater legs 54, 56 above andbelow the crucible 16 respectively at their desired heights. A topheater post 66 having a upper portion diameter of about 2 inches, anupper portion length of about 1.5 inches, a lower portion diameter ofabout 1.5 inches, and a lower portion length of about 15.5 inches isbelieved to be suitable for a four station furnace having a heatermatrix with two legs of two heaters each and the above describedcrucibles for growing single crystals of calcium fluoride. Suitableranges for the dimensions of the top heater post 66 are believed to bean upper portion diameter of from about 2 inches to about 6 inches, anupper portion length of from about 1 inch to about 3 inches, a lowerportion diameter of from about 1 inch to about 3 inches, and a lowerportion length of from about 6 inches to about 20 inches. A bottomheater post 67 having a upper portion diameter of about 2 inches, anupper portion length of about 1.5 inches, a lower portion diameter ofabout 1.5 inches, and a lower portion length of about 1.25 inches isbelieved to be suitable for a four station furnace having a heatermatrix with two legs of two heaters each and the above describedcrucibles for growing single crystals of calcium fluoride. Suitableranges for the dimensions of the bottom heater post 67 are believed tobe an upper portion diameter of from about 2 inches to about 6 inches,an upper portion length of from about 1 inch to about 3 inches, a lowerportion diameter of from about 1 inch to about 3 inches, and a lowerportion length of from about 3 inches to about 12 inches.

As best shown in FIGS. 3 and 4, the heaters 20, 22 and heater legs 54,56 are electric resistance heaters and are preferably formed from asingle sheet or bar of material. The heater legs 54, 56 preferablycomprise graphite or other suitable materials such as, for example, acarbon—carbon composite, a polycarbon composite, or silicon carbide.

In the illustrated embodiment each heater leg 54, 56 has two of theheaters 20, 22 formed therein. Each heater leg preferably has a centralportion 68, 70 connecting the heaters 20, 22 and end portions 72, 74extending from opposite sides of the heaters 20, 22. The end portions72, 74 are adapted to be connected to the bus bars 58, 60 such as, forexample, providing openings for the passage of fasteners therethrough.It is noted that additional or fewer heaters 20, 22 can be formed in asingle heater leg 54, 56 as discussed in more detail hereinafter.Graphite heater legs 54, 56 having a thickness of about 5 mm, a totallength of about 29.5 inches, heater outer diameters of about 8.25inches, a central portion length of about 4{fraction (6/8)} inches, endportion lengths of about 4⅛ inches, and central and end portion widthsof about 3 to about 5 inches are believed to be suitable for a fourstation furnace having a heater matrix with two legs of two heaters eachand the above described crucibles for growing single crystals of calciumfluoride. Suitable ranges for the dimensions of the heater legs 54, 56are believed to be a thickness of from about 2 mm to about 18 mm, atotal length of from about 8 inches to about 108 inches, heater outerdiameters of from about 6 inches to about 40 inches, a central portionlength of from about 4 inches to about 10 inches, end portion lengths offrom about 4 inches to about 8 inches, and central and end portionwidths of from about 2 inches to about 6 inches.

Each heater 20, 22 preferably has body with a generally circular outerperiphery and a constant thickness. The body is provided with gaps orslots 76, 78 which produce a “zig-zag” circuit of current flow paths.The flow paths are preferably formed by arcuate or curvedcircumferentially extending sections 80, 82 connected at their ends. Inthe illustrated embodiment, there are five sections 80, 82 but a greateror lesser number of sections 80, 82 can be utilized. A first half of thebody has two separate current flow paths which extend from the first oroutermost section 80 a, 82 a to the fifth or innermost section 80 e, 82e and a second half of the body two separate current flow paths whichextend from the innermost section 80 a, 82 a to the outermost section 80e, 82 e. Note that the first and second halves of the body are isolatedfrom one another except at the fifth or inner most section 80 a, 82 a.Preferably, the two current flow paths are connected along their lengthsat intermediate points between the innermost section 80 a, 82 a to theoutermost section 80 e, 82 e. The current flow paths of illustratedembodiment are connected at the junction between the second sections 80b, 82 b and the third sections 80 c, 82 c and at the junction betweenthe fourth sections 80 d, 82 d and the fifth sections 80 e, 82 e.

The heaters 20, 22 within each heater matrix 50, 52 can be the same ordifferent depending on the needs of the crystal growth furnace 10. Eachof the heaters 20, 22 within a heater matrix 50, 52 can be the same,that is, have the same resistance such that for any givenamperage/voltage, the temperature will be the same at the individualgrowth stations 14. The heaters 20, 22 within a heater matrix 50, 52 canalternatively each have a different resistance such that for any givenamperage/voltage, the temperature will be higher or lower at the variousindividual growth stations 14. In the illustrated embodiment, each ofthe top heaters 20 within the top heater matrix 50 are the same and eachof the bottom heaters 22 within the bottom heater matrix 52 are thesame.

Likewise, the top and bottom heaters 20, 22 of each individual growthstation 14 can be the same or different depending on the needs of thegrowth stations 14. In the illustrated embodiment, the top and bottomheaters 20 within each growth station 14 are different.

As best shown in FIG. 3, each top heater 20 is designed to provide, atany given amperage/voltage, a homogeneous thermal environment across thetop of the crucible 16. Accordingly, the sections 80 a-80 e each havesubstantially the same width. The dimensions of the top heater 20 dependon the thermal environment desired and the amperage/voltage desired tobe supplied. A graphite top heater having a thickness of about 5 mm, anouter diameter of about 8.25 inches, an inner diameter of about 1 inch,gap widths of about ⅛ inch, and section widths of about ⅝ inch isbelieved to be suitable for the above described crucibles for growingsingle crystals of calcium fluoride. Such top heaters preferably operatewith total power of about 20 kw (for two legs of two heaters) and have atotal resistance of about 0.0508 ohms (for two legs of two heaters),therefore there is about 627 amps and 32 volts secondary (going to thebus) and about 85 amps and 240 volts primary (going to the transformer (step down factor of 2.5)). Other dimensions, resistances, and power willbe obvious to one skilled in the art to obtain desired thermalenvironments.

As best shown in FIG. 4, each bottom heater 22 is designed to provide,at any given amperage/voltage, a radial thermal gradient across thebottom of the crucible, that is, a thermal environment with anincreasing temperature in the radial direction, from the center of thecrucible 16 to the outer periphery of the crucible 16. The innermostsection 82 e of the heater 22 is the coldest section and the temperatureof each of the sections 82 a-82 e gradually increases to the outermostsection 82 a which is the hottest section. This temperature gradientensures that the origin of the crystal growth will be at one positionwithin the crucible 16 and that location is at the central axis of thecrucible 16.

The dimensions of the bottom heater 22 depend on the thermal environmentdesired and the amperage/voltage desired to be supplied. A graphitebottom heater having a thickness of about 5 mm, an outer diameter ofabout 8.25 inches, an inner diameter of about 1 inch, gap widths ofabout ⅛ inch, and increasing section widths of about ⅜ inch, about{fraction (4/8)} inch, about ⅝ inch, about {fraction (6/8)} inch, andabout ⅞ inch is believed to be suitable for the above describedcrucibles for growing single crystals of calcium fluoride. Such bottomheaters preferably operate with total power of about 20 kw (for two legsof two heaters) and have a total resistance of about 0.06895 ohms (fortwo legs of two heaters), therefore there is about 540 amps and 37 voltssecondary (going to the bus) and about 83 amps and 240 volts primary(going to the transformer (step down factor of 2.5)). Other dimensions,resistances, and power will be obvious to one skilled in the art toobtain desired thermal environments.

As best shown in FIG. 1, the heaters 20, 22 of each heater matrix 50, 52are connected to a single power source 84, 86 so that at any given powerlevel, the temperature of all the heaters 20, 22 within the heatermatrix 50, 52 are fixed and may or may not be the same through out theheater matrix 50, 52 depending on the individual design of the heaters20, 22. Preferably, the heater matrices 50, 52 each have a separatepower source 84, 86. A first power source 84 controls the upper heatermatrix 50 and a second power source 86 controls the lower heater matrix52 so that the temperature of the heater matrices 50, 52 relative toeach other can be varied. The separate power sources 84, 86 of theheater matrices 50, 52 are preferably controlled by a single controller88. The power sources 84, 86 and the controller 88 can be conventional.

FIG. 6 illustrates a heat treatment or vacuum furnace 100 according to asecond embodiment of the present invention wherein like referencenumbers are used for like structure. The crystal growth furnace 100illustrates that additional levels or layers of individual growthstations 14 can be utilized. In the illustrated embodiment there arethree levels of four individual growth stations 14 so that the crystalgrowth furnace 100 has twelve individual growth stations 14. Each heatermatrix 50, 52 includes six heater legs 54, 56 connected in parallel witheach leg having two heater 20, 22 to accommodate the twelve individualgrowth stations 14. While the illustrated embodiment has three levels,it is noted that a greater or lesser number of layers can be utilized.It is also noted that the features of the second embodiment can beutilized alone or in combination with each of the features of each ofthe other disclosed embodiments.

FIG. 7 illustrates a heat treatment or vacuum furnace 200 according to athird embodiment of the present invention wherein like reference numbersare used for like structure. The crystal growth furnace 200 illustratesthat the top heater legs 54 can extend parallel to the bottom heaterlegs 56 rather than perpendicular as in the first embodiment.Accordingly, the bus bars 58, 60 are parallel on one side rather thanperpendicular on different sides as in the first embodiment. In thisembodiment the top heater legs 54 have a larger length than the bottomheater legs 56. It is also noted that the features of the thirdembodiment can be utilized alone or in combination with each of thefeatures of each of the other disclosed embodiments.

FIG. 8 illustrates a heat treatment or vacuum furnace 300 according to afourth embodiment of the present invention wherein like referencenumbers are used for like structure. The crystal growth furnace 300illustrates that there can be additional heater legs 54, 56 and that theheater legs 54, 56 can have addition heaters 20, 22. In the illustratedembodiment, there are three heater legs 54, 56 each having three heaters20, 22 so that the crystal growth furnace 300 has one level of nineindividual growth stations 14. While the illustrated embodiment has onelevel, three heater legs 54, 56, and three heaters 20, 22 in each heaterleg 54, 56, it is noted that a greater number of layers can be utilizeda greater or lesser number of heater legs 54, 56 in each layer can beutilized, and a greater of lesser number of heaters 20, 22 in eachheater leg 54, 56 can be utilized. It is also noted that the features ofthe fourth embodiment can be utilized alone or in combination with eachof the features of each of the other disclosed embodiments.

It is noted that the fourth embodiment can be particularly advantageouswherein the heaters 20, 22 are different within the heater matrices 50,52 to form different thermal environments on different individual growthstations 14. This allows the furnace to be utilized to grow crystalsusing a continuous process technique. In the continuous processtechnique, material is loaded into containers at one end of the crystalgrowth furnace 300 and the material passes through a number ofindividual growth stations 14 having different thermal gradients. Forexample, the material can pass through individual growth stations 14which are adapted to separately heat up starting material, grow thecrystal, and cool down the crystal to room temperature.

FIG. 9 illustrates an alternative heater leg 90 wherein like referencenumbers are used for like structure. The heater leg 90 illustrates thatthe heater legs can have shapes other than the above described linearconfiguration (FIGS. 3 and 4), such as the illustrated circularconfiguration. In the illustrated embodiment, the heater leg 90 isgenerally circular and has six of the heaters 20, 22 incorporatedtherein. It is noted that a greater or lesser quantity of the heaters20, 22 can be utilized and the heaters 20, 22 can have differentlocations than illustrated. It is also noted that the alternative heaterlegs 90 can be utilized alone or in combination with each of thedisclosed embodiments. The circular arrangement of the heater leg 90 canbe particularly advantageous when a plurality of circular heater legs 90of various diameters are coaxially arranged.

Although particular embodiments of the invention have been described indetail, it will be understood that the invention is not limitedcorrespondingly in scope, but includes all changes and modificationscoming within the spirit and terms of the claims appended hereto.

What is claimed is:
 1. A crystal growth furnace having multiple crystalgrowing stations comprising: a crucible at each growth station adaptedto grow crystals therein; a power source; a first heater matrix havingat least one matrix leg, each matrix leg including at least tworesistance heaters, each of the heaters having a first end and a secondend, the first end of at least one of the heaters electrically connectedto the power source, so that electrical interconnections between saidtwo resistance heaters in the first heater matrix are in parallel andare adapted to transmit power simultaneously, at any given time, to allthe resistance heaters in the first heater matrix; each crucible in eachgrowth station having at least one of the resistance heaters associatedtherewith so that each crucible is heated by a resistance heater; andthe power source being adapted for powering said two resistance heaterssimultaneously, at any given time, whereby the crystal growth furnace isadapted to grow crystals uniformly and simultaneously at each growingstation.
 2. The crystal growth furnace according to claim 1, whereinsaid resistance heaters are located below said crucibles.
 3. The crystalgrowth furnace according to claim 2, wherein said resistance heaterscomprise graphite sheets having slots to define electrical arc-shapedflow paths having resistances for obtaining thermal gradients, said flowpaths having cross-sections of varying size to obtain a thermal gradientwhich is cooler towards the center and warmer radially outwardly therebycausing crystals to grow from the center outwardly.
 4. The crystalgrowth furnace according to claim 3, which further includes a secondheater matrix including at least two resistance heaters, at least one ofsaid heaters being above each of said crucibles, said resistance heatersbeing connected in series or parallel.
 5. The crystal growth furnaceaccording to claim 4, wherein said resistance heaters in said secondmatrix are adapted to provide a homogeneous thermal environment acrosstops of said crucibles.
 6. The crystal growth furnace according to claim5, wherein said resistance heaters in said second matrix comprisegraphite sheets having slots to define arcuate electrical flow pathshaving resistances for obtaining thermal environments, said flow pathshaving cross-sections of generally constant size to obtain a homogeneousthermal environment.
 7. The crystal growth furnace according to claim 4,wherein said individual growing stations each have an insulatingcontainer generally surrounding said crucible and a single thermalchamber enclosing all the growing stations in order to havesubstantially the same environment and temperature around eachcontainer.
 8. The crystal growth furnace according to claim 1, whereinsaid first heater matrix includes at least two legs of at least two ofsaid resistance heaters and said legs are electrically connected inparallel, each resistance heater in said first matrix being locatedbelow a crucible.
 9. The crystal growth furnace according to claim 8,wherein all of said resistance heaters in the first matrix comprisegraphite sheets having slots to define arcuate electrical flow paths,the flow paths having cross-sections going from larger to smaller fromthe center radially outwardly so that a gradient is created from coolerto hotter from the center outwardly and thereby causing crystalformation from the center radially outwardly.
 10. The crystal growthfurnace according to claim 9, wherein said second heater matrix includesat least two legs of at least two of said resistance heaterselectrically connected in series and said legs are electricallyconnected in parallel.
 11. The crystal growth furnace according to claim10, further comprising a second power Source connected to the secondheater matrix.
 12. A crystal growth furnace having multiple crystalgrowing stations comprising: a crucible at each growing station adaptedto grow crystals therein; a first power source; a first heater matrixincluding at least two legs, each matrix leg having a first end and asecond end, each of the first ends of the matrix legs electricallyconnected to the first power source, each of the second ends of thematrix legs electrically connected together. thereby causing the matrixlegs to be electrically connected in parallel, each of said legs havingat least two resistance heaters, each of the heaters having a first endand a second end, the first end of both of the heaters electricallyconnected to the first power source, the second end of said one heaterelectrically connected to the first end of a second one of the heaters,thereby causing the heaters to be electrically connected in parallel sothat electrical interconnections between the resistance heaters in thefirst heater matrix are adapted to transmit power simultaneously, at anygiven instance, to all the resistance heaters in the first heatermatrix, each of said individual growing stations having at least one ofsaid resistance heaters of said first heater matrix associated therewithand located near said crucible; a second power source; a second heatermatrix including at least two legs, each matrix leg haying a first endand a second end, each of the first ends of the matrix legs electricallyconnected to the second power source, each of the second ends of thematrix legs electrically connected together, thereby causing the matrixlegs to be electrically connected in parallel, each of said legs havingat least two resistance heaters, each of the heaters having a first endand a second end, the first end of one of the heaters electricallyconnected to the second power source, the second end of said one heaterelectrically connected to the first end of a second one of the heaters,thereby causing the heaters to be electrically connected in series sothat electrical interconnections between the resistance heaters in thesecond matrix are adapted to transmit power simultaneously, at any giventime, to the resistance heaters in the second heater matrix, whereinsaid second heater matrix is separate from said first heater matrix andeach of said individual growth stations has at least one of saidresistance heaters of said second heater matrix associated therewith andlocated near said crucible; the first power source adapted fortransmitting power to all of the resistance heaters connected to thefirst heater matrix simultaneously, at any given time; and the secondpower source adapted for transmitting power to all of the resistanceheaters connected to the second heater matrix simultaneously, at anygiven time, whereby the crystal growth furnace is adapted to growcrystals uniformly and simultaneously at each growth station.
 13. Thecrystal growth furnace according to claim 12, wherein said resistanceheaters of said first heater matrix is located below said crucibles andare adapted to provide a thermal gradient across bottoms of saidcrucibles.
 14. The crystal growth furnace according to claim 13, whereinsaid resistance heaters of said first heater matrix comprise graphitesheets having slots to define arcuate electrical flow paths havingresistances for obtaining thermal environments, said flow paths havingcross-sections of varying size from larger to smaller to create atemperature gradient from cooler to hotter from the center radiallyoutwardly whereby crystals grow from the center outwardly.
 15. Thecrystal growth furnace according to claim 14, wherein said resistanceheaters of said second heater matrix are adapted to provide ahomogeneous thermal environment across tops of said crucibles.
 16. Thecrystal growth furnace according to claim 14, wherein said resistanceheaters in said second matrix comprise graphite sheets having slots todefine electrical flow paths having resistances for obtaining thermalenvironments, said flow paths having cross-sections of generallyconstant cross section to obtain said homogeneous thermal environment.17. The crystal growth furnace according to claim 16, wherein all of theresistance heaters in the first matrix are substantially the same andall of the resistance heaters in the second matrix are substantially thesame so that all growth stations are heated and cooled to the sametemperature at the same time.
 18. The crystal growth furnace accordingto claim 17, further comprising separate first and second power sourcesconnected to said first and second heater matrices respectively.
 19. AThe crystal growth furnace according to claim 17, further comprising asingle thermal chamber covering all of the growing stations in order toprovide a uniform thermal environment whereby the crystal growingstations may grow more uniform crystals simultaneously at each growingstation.
 20. The crystal growth furnace according to claim 16, whereinsaid first and second heater matrices are adapted to provide differentthermal environments for said individual growing stations.
 21. A crystalgrowth furnace having multiple crystal growing stations comprising: acrucible at each growth station adapted to grow crystals therein; apower source; a first heater matrix having at least one matrix leg, eachmatrix leg including at least two resistance heaters, each of theheaters having a first end and a second end, the first end of at leastone of the heaters electrically connected to the power source, so thatelectrical interconnections between said two resistance heaters in thefirst heater matrix are in series or parallel and are adapted totransmit power simultaneously, at any given time, to all the resistanceheaters in the first heater matrix; each crucible in each growth stationhaving at least one of the resistance heaters associated therewith sothat each crucible is heaved by a resistance heater; and the powersource being adapted for powering said two resistance heaterssimultaneously, at any given tame, whereby the crystal growth furnace isadapted to grow crystals uniformly and simultaneously at each growingstation.